1. Field of Invention
This invention relates to the recovery of light in projection systems that might otherwise be wasted.
2. Description of the Related Art
Projection displays work by projecting light onto a screen. The light is arranged in patterns of colors or brightness and darkness or both. The patterns are viewed by a viewer who assimilates them by associating the patterns with images with which the viewer may already be familiar such as characters or faces. The patterns may be formed in various ways. One way is by modulating a beam of light with a stream of information.
Polarized light may be modulated by filtering it with polarized filters. An LCD imager may be used to perform the modulation in LCD-type projection displays. The LCD imager may include pixels that may be modulated by altering their polarization to either match the polarization of the incident light or differ from it. The pixels will pass light, in general, if their polarization matches the polarization of the incident light.
The light input to the LCD imager is polarized such that when the LCD pixels are modulated the polarization of the selected pixels is changed, and when the light output from the imager is analyzed by another polarizer, the selected pixels will be darkened.
If the polarization of the pixels is modulated with information, such as a pattern with which a viewer may be familiar, the information may be projected onto a screen as the presence or absence of light. When the pixels are modulated with information forming a pattern with which a viewer may be familiar, a viewer may recognize the pattern projected onto the screen.
Only half of the light from the source will be of the correct polarization to input to the LCD imager after the light is polarized. The other half will be of the incorrect polarization and hence unusable directly. It would be desirable if light of the wrong polarization could be recovered and used by converting its polarization to the correct type.
Various schemes have been developed to convert the incorrectly polarized light to the correct polarization so that it too may be used. A common scheme uses an array of lenses and an array of polarization beam splitters (PBS). These systems usually focus the output of a light source with a parabolic reflector such that light is nearly parallel. The beam is split into many sections by a lens array and each section is refocused by another lens array into the PBS array. The light output from the PBS array will then be linearly polarized and focused into the target.
A straight light pipe (SLP) may be used to homogenize a beam of light focused by, e.g. an elliptical reflector as described in U.S. Pat. No. 6,139,157. Multiple images will be formed when viewed from the output of the SLP due to the multiple reflections of the focused light by the sidewalls of the SLP. As a result, the focused light and its reflections can be imaged onto the lens array and PBS array in a manner similar to the case of the parabolic reflector.
Light pipe 2 shown in FIG. 1 may be used with a 1:1 dual paraboloid coupling system. Light of an improper polarization may be recovered inside light pipe 2. Light pipe 2 may be either hollow or solid, and has an opening 4 at an input end 6 of light pipe 2, as shown in FIG. 1A. The remainder of input end 6 is coated with a reflective surface 16. A polarizer 8 is placed at an output end 10 of light pipe 2.
The area of opening 4 is generally limited to a fraction of the area of a cross-section of light pipe 2. The area of opening 4 is commonly less than or equal to half of the area of a cross-section of light pipe 2. A fraction of one half will be used in the following descriptions.
Randomly polarized light 18 enters light pipe 2 at opening 4 and passes to polarizer 8. Light of a first polarization 12 is transmitted at polarizer 8 while light of a second polarization 14 is reflected back toward input end 6 of light pipe 2. A portion of the light of second polarization 14 exits opening 4 at input end 6 of light pipe 2, while the remainder is reflected and randomized by reflective surface 16 and directed toward polarizer 8 again.
If any of the light is reflected at polarizer 8 again, however, it will repeat the reflection process again and another part of the light will be recovered. The process will repeat itself over and over until the light is either dissipated or transmitted, and contribute to the final output of light of first polarization 12 at output end 10.
One problem with this configuration arises from the fact that about half of the reflected light of second polarization 14 is lost through opening 4 at input end 6 of light pipe 2 during each pass. Therefore, if an efficiency of polarizer 8 is 50%, the reflectivity of reflective surface 16 at input end 6 is 100%, and randomly polarized light 18 is truly randomly polarized, the output from the first pass will be about one-half of the input light. The other half of the light will be reflected back to input end 6, where half of that will pass through opening 4 and the other half will be reflected by reflective surface 18. Only the half reflected by reflective surface 18 will reach polarizer 8 again, and only half of that will exit as light of first polarization 12. The overall second pass efficiency will thus be xe2x85x9. The output for the third pass will be {fraction (1/32)}, and so on. The sum of this geometric series is ⅔, which is 66%.
Reflective surface 16 may include a wave-plate 20 so that a majority of light of second polarization 14 is rotated to light of first polarization 12, rather than randomized, and consequently transmitted through polarizer 8. Reflections inside light pipe 2 may depolarize light of second polarization 14 to a certain extent. Even so, wave-plate 20 may ensure a higher proportion of light exits polarizer 8 on the second pass.
Although these systems has been used commercially, the cost of the components is high and they require critical alignments and optical designs. As a result, there is a need for a system to perform polarization conversion with high efficiency, simple configurations and lower costs.
In one embodiment the invention includes a polarization recovery system using light pipes with a reflector having a first and a second focal points. A source of electro-magnetic radiation is disposed proximate to the first focal point of the reflector to emit rays of light that reflect from the reflector and converge substantially at the second focal point. A light pipe having an input surface with a first transmissive portion disposed proximate to the second focal point and a second reflective portion disposed distal to the second focal point and an output surface collects and transmits substantially all of the light. The output surface has a radius of curvature substantially equal to a length of the light pipe, a first focal area proximate to the first transmissive portion and a second focal area proximate to the second reflective portion. A polarizer applied to the output surface transmits the light of a first polarization and reflects the light of a second polarization toward the second reflective area, and the light of the second polarization is substantially reflected by the second reflective area toward the output surface.